1. Field of the Invention
This invention relates to data input-output terminal systems, and more particularly to a system having a device controller to which displays or other I/O devices are connected by a transmission system such as individual coaxial lines to the respective devices.
2. Description of the Prior Art
Display systems are known which include a controller disposed between a local or remote host computer and a cluster of keyboard display terminals. One such system is shown and described in U.S. Pat. No. 4,271,479. In the system described in that patent, a controller including a processor, storage, and a device cluster adapter for connection of the controller to a plurality of keyboard displays via individual coaxial cables is described. Each individual display includes a cathode ray tube refresh buffer memory in which data in the form of coded display characters and display control codes is written by the remote controller. The displays are dependent upon the controller for interpretation of keystroke data and the writing of that data back at the proper place in the refresh memory of the display. The coaxial cables may be, for example, up to 5,000 feet long and although capable of high frequency bit operation, are serial transmission devices. Systems of this kind have been known for a number of years and may include, for example, an IBM 3274 Control Unit and a plurality of IBM 3278 or IBM 3279 Display Stations. Other display terminals are known which include a microprocessor and memory which in addition to providing a coded image of the display screen, includes working space for inbound and outbound traffic. An example is the IBM 8775 Display Terminal. "Intelligent" output terminals, such as the IBM 3287 and 3289 printers, are known which have buffers in half duplex association with the controller and therefore have storage while is logically part of the controller in a limited sense. (IBM is a registered trademark of International Business Machines Corporation).
It has also been known that it would be desirable to enable keyboard display devices to support concurrent independent sessions on the display terminal. One such arrangement is shown and described in the IBM Technical Disclosure Bulletin, Vol. 21, No. 4, September 1978, pp. 1524, 1525. Thus, there has been an impetus for the development of display systems which can handle more complex operations in the display device itself. This has been made more practical by the ever increasing power and decreasing cost of microprocessors suitable for incorporation in such devices. With these developments in the art, there has arisen, then, the occasion of greater volume and variety of traffic over the transmission link in display systems of the controller-remote display device architecture of the general kind shown in the aforesaid U.S. Pat. No. 4,271,479. Accordingly, it has become desirable to provide ways of handling this traffic more efficiently.